Leachable ceramic cores

ABSTRACT

Ceramic cores having at least sufficient zinc oxide to provide essential continuity throughout the structure are provided. Such cores are included in metal castings from which they are leached under acidic or alkaline conditions to provide cavities within the castings.

United States Patent 11 1 Bailey Apr. 17, 1973 1 LEACHABLE CERAMIC CORES [56] References Cited [75] Inventor: Joseph T. Bailey, Chattanooga, UNITED STATES PATENTS erm 3,232,771 2/1966 Pearce ..l06/38.35

[73] Assignee: American Lava Corporation, Chat- 2,502,337 3/1950 Moir ..l64/l32X tanooga, Tenn. 2,362,875 11/1944 Zahn ..l64/132 [22] Filed: May 27,1971

us. Cl. ..l64/132 1m. (:1. ..B22d 29/00 Field of Search 164/132, 369',

Primary Examiner l. Spencer Overholser Assistant Examiner-John E. Roethel Att0meyl(inney, Alexander, Sell, Steldt & Delahunt [57] ABSTRACT Ceramic cores having at least sufficient zinc oxide to provide essential continuity throughout the structure are provided. Such cores are included in metal castings from which they are leached under acidic or alkaline conditions to provide cavities within the castings.

3 Claim, No Drawings LEACHABLE CERAMIC CORES This is a division of application Ser. No. 739,156 filed June 24, 1968.

This invention relates to novel refractory cores and molds for use in ferrous and nonferrous metallurgy. In particular this invention relates to leachable refractory cores and molds consisting essentially of zinc oxide although including, before calcining and firing, varying amounts of organic and inorganic additives and/r diluents of which inorganic additives are generally retained after calcination and firing.

in the production of complex mechanical parts and pieces it is frequently necessary to provide cavities or internal passages of relatively complex shape and quite accurate dimensions. Some internal passages may be formed by drilling or other conventional metal-working operations, but use of ceramic cores is preferred practice because of the economies of processing possible. Furthermore, precise internal configurations may be required in positions which are either prohibitively difficult or impossible to machine by conventional methods but are readily produced using leachable cores. illustrations are internal undercuts, flanges, bosses, or threads, complex internal air foil shapes, curving or helical passageways, etc.

In an illustrative process, a preformed leachable ceramic core of the desired configuration and dimensions is positioned in an injection molding die of the external shape etc. desired, and while suitably supported, is surrounded byja wax pattern of the object to be cast together 'with suitable sprues. ,A refractory shell,

.suitablyof conventional investment composition, is

then formed around the wax and leachable eoreassembly by positioning in a suitable mold and pouring the investment therearound with conventional debubbling, etc. Alternatively the shape may be dipped repeatedly in the investment composition. Afterdrying the invested mold, wax is melted and partially drained from the mold and residual wax is then burned out. The

desired metal is poured into the resultant cavity in the hot mold by gravity, centrifuging or other process. After solidification of the casting, the shell mold is removed from the metal casting. Suitable connections I are made to the ceramic core and it is removed by leaching to leave cavities, channels, etc; as desired. The manner'of using leachable cores will be apparent to those skilled in the art.

Leac hable ceramic compositions used heretofore have usually been soluble only in hydrofluoric acid and/or hot caustic baths (i.e.,,sodium hydroxide). it will be recognized that not all alloys can survive under such strenuous chemical conditions. In fact, hot fused caustics are known to have deleterious effects on many ,metals. Less strenuous conditions for leaching would be very desirable 'to permit more widespread applicability of leachable cores to difficult problems of fabrication. 1

It is an aim of this invention to provide refractory cores which tolerateconditions for casting metals, for example atmold temperatures l000-l 200 C., andare dissolvable at relatively rapid ratesunder conditions of such moderate acidity that there is no attack of metals such as aluminum, lead, iron, copper, nickel, silver, brass, bronze, chrome-nickl alloys, lead-tin alloys, lead-bismuth alloys, or high temperature alloys such as Haynes Stellite Alloy Na, 31 of the Union Carbide Corp; Other aims and objects of the invention will become evident on reading the present disclosure.

[t is found that zinc oxide is admirably suited to preparing moldable compositions and thus in achieving aims of this invention, when it is compounded in finely divided or powdered form with binders such as starches, waxes, polyvinyl alcohol, thermoplastic resins, etc. in amounts according to methods of molding to be employed. Optionally, up to 40 percent or more of inert filler or diluent, such as zircon in relatively fine sizes, and/or up to 25 percent of a bulk filler to promote porosity such as powdered organic materials, wood'flour, or other cellulosic filler, etc. are included in the composition. The amount of filler must not be so great that it will react completely with zinc oxide under conditions either of firing the core or during casting of metal. For example, zircon reacts at elevated temperatures e.g., l 100 C., with zinc oxide to give zirconia and zinc silicate. Compositions including weight percentages above 40-50 percent of zircon may therefore be so reacted above l000 C. that no zinc oxide remains and the core is no longer leachable.

The composition is formed into the desired shapes by methods of the ceramic art such as extrusion, casting, dry pressing, isostatic pressing, injection molding, etc. The composition, as noted, is formulated in detail on the basis of the method of molding which is employed. Thus for dry or isostatic pressing, 2 to 5 percent of starch, wax, polyvinyl alcohol, etc. are incorporated and the composition granulated and dried to less than 1 percent water content. For extrusion, 4 to 7 percent of the above binders are incorporated together with 15 to 22 percent water. For injection molding 17-25 percent thermoplastic binders are included without water. The amounts of the various fillers, binders, etc. are adjusted to alter thermal expansion coefficients, shrinkage on firing and hardness and porosity of the finished core as will be evident to those skilled in ceramic arts.

In sintering ZnO, air is advantageously forced into the kiln to maintain oxidizing conditions. if fired to temperatures of l050 to 1350 C. zinc oxide cores are quite hard and, when additional finishing is needed, must be-shaped and worked using abrasives. They are soft fired or underfired in a current of air to about 800 to 1 100 C., and are then more easily worked, for example, by metal working tools.;As noted reactions of fillers may alter properties.

such hard cores are dissolved at relatively rapid rates in 20 percent aqueous acetic acid at C. They may be dissolved, but somewhat moreslowly in more dilute (about 10 percent aqueous) acetic acid.

Other materials which may be used for leaching the zinc oxide cores, i.e., as leacharlts, are otherdilute acids, e.g., sulphuric, nitric, hydrochloric, phosphoric. Dilute alkalis, such as 3 3 percent potassium hydroxide at C., may also be used but solution is much slower than in acids.

bined zinc oxide in the fired core to provide essentially continuous paths for dissolution of the core. The lower concentrations of zinc oxide and more porous structures are naturally somewhat cheaper for larger cores. A relatively cheap filler such as zircon may be used and shrinkage may be reduced thereby. As noted above the amount must be sufficiently low so that at no temperature encountered by the core is the zinc oxide phase rendered inaccessible to leachants. It will be evident that amounts of fillers incorporated must be sufficiently low that there is no opportunity for agglomeration into clinker-like masses which will not pass out of the cavities remaining after leaching of cores.

The zinc oxide which is used is of commercially available grades and may be either American Process or French Process. Particle sizes may range from 0.4 to 0.8 microns mean particle size for U.S.P. Grades (normally used in cosmetics, pharmaceuticals, etc.) to ceramic grades having mean particle sizes of up to 2 to 5 microns. Typical purities range from 99.8% ZnO for vU.S.P. grades to 99.4% ZnO for ceramic grades. The

U.S.P. grades are advantageous from purity and reactivity standpoints but the ceramic grades offer advantages of easier forming and lower shrinkage during sintering. It is sometimes desirable to blend materials of the two grades together in order to benefit from the desirable properties of both. to blend materials of the two grades together in order to benefit from the desirable properties of both.

The composition of zinc oxide and whatever inorganic filler is first ball-milled to effect particle size reduction when desired. It is then mixed with organic binders and any liquids, plasticizers, etc. in suitable equipment such as a muller-type mixer to a desired consistency which is a thick dough for extrusion into rods, tubes, etc. Wood flour or other organic fillers are incorporated in the same equipment or, the composition is mixed with 17 to 25 percent of thermoplastic resins, for example, for the injection molding of cores suitable to provide internal passages for complex air foil shapes for turbine engines.

Unfired greenshapes are formed by a selected convenient method, dried and then fired at temperatures in the range of 800to about 1 100 C. when a porous core body is desired or at higher temperatures up to about 1350 C. when a hard body impervious to water absorption is desired. An initial firing to destroy all or part of the organic materials may be employed if desired. Shrinkages of the order of to percent occur and are carefully controlled. Allowance therefor is made in shaping the piece prior to sintering. After cooling the leaehable core body of zinc oxide is further finished if necessary by diamond grinding, et'c.

Leachable core bodies are used, for example, in hollow gas turbine blades and stator vanes, pitot tubes, temperature probes, fuel controls, heat exchangers.

EXAMPLE 1 A green, i.e., unfired, zinc oxide cylindrical tube is formed by extruding a blend mixed in a muller-type mixer of 100 parts of U.S.P. grade zinc oxide (average particle size 0.4 to 0.7 microns), 3 parts corn starch and 2 parts microcrystalline wax emulsion and 12 parts of water. The green tube is 2.0 and 5.3 mm. in internal and external diameters respectively. This provides a tube which is impervious to dye or water penetration, and has respective external and internal diameters of 4.4 and 1.6 mm. after firing in a current of air for six hours at ll0O C. The respective diameters of a tube fired for 6 hours at lO0O C. are 4.4 and 1.6 mm. and the tube absorbs up to about 1 percent of water. Shrinkage is less and the diameters are respectively 4.6 and 1.8 mm. after firing at 900 C. This tube absorbs up to about 4.5 percent of water. Solid rods are extruded at a diameter of 6.3 mm. by the same procedure fired as above with similar shrinkage and absorption being obtained. After firing, at indicated temperatures rod diameters are 5.4 mm. (900 C.), 5.2 mm. (1000 C.), 1

and 5.1 mm.(l100C.).

Tubes and rods in lengths of 55 to 65 mm. are placed in an aqueous solution (20 percent of acetic acid) at -98 C. with following results:

Approximate time Tubes of these types are molded in a wax block and invested by usual procedures. After normal burnout and casting of bronze, the investment is removed and a bronze block in the shape of the wax block is obtained including the ceramic tubes as cores. These cores are removed by leaching with hot 20% acetic acid. Rods used as cores are dissolved much more slowly because of the smaller end areas available for solution. The cavities in the casting replicate the cores employed.

Samples of core pieces are also dissolved in a 10 percent aqueous solution of acetic acid over somewhat longer times. Samples of rods are dissolved in less than 20 minutes in an ultrasonic bath of 20 percent aqueous acetic acid at same solution concentration and temperature.

EXAMPLE 2 The procedure of Example 1 is repeated except that a blend of 95 parts U.S.P. ZnO and 5 parts wood flour is extruded into rods and tubes. Data are tabulated below. I

Approximate time Water required Firing absorp- Sample for decom temp. tion OD lD weight position (0) Sample (1%) (mm.) (grams) (min.) 900 Tube 9.0 4.7 1.8 3.4 l3 900 Rod 9.0 5.6 5.4 30 1000 Tube 47 4.5 L7 3.4 l2 I000 Rod 4.7 5.2 5.3 20 I Tube 2.0 4.4 l.(\ 3.4 13 H00 Rllkl 2.0 5.1 i 5.4 .17

These tubes are employed as cores as described in Example I for silver casting. Leaching by dilute hydrochloric acid yields a silver block having cavities replicating the outer surfaces of tubes.

EXAMPLE 3 The procedure of Example 1 is repeated except that a blend of 90 parts U.S.P. zinc oxide and l0 parts wood flour is extruded into rods and tubes. The data are tabulated below.

Approximate time Water required Firing absorp- Sample for decomtemp. tion OD ID weight position (C) Sample (mm.) (grams) (min.) 900 Tube 20 4.7 1.8 2.8 15 900 Rod 20 5.7 4.4 25 1000 Tube 4.5 1.7 2.8 17 1000 Rod 10 5.2 .3 25 1100 Tube 7 4.3 1.6 2.8 19 1 100 Rod 7 5.0 4.4 30

These tubes and rods are employed as cores as in Example 1 and are dissolved more readily from the metal block.

EXAMPLE 4 Sample tubes are produced as described in Example 2 and fired at ll0O C. for 6 hours. Water absorption on the fired tubes is almost 2.0 percent and tubes, 4.4 mm. OD by 1.6 mm. ID by 55 to 65 mm. long and weighing almost 3.4 grams, are placed in heated (909 8 C.) 20 percent aqueous solutions of hydrochloric acid, nitric acid, sulfuric acid, and phosphoric acid respectively. In all cases, tubes are completely decomposed in less than 25 minutes.

Another tube is placed in a heated l l0-l C.) 33 percent aqueous solution of sodium hydroxide. Decomposition in this solution requires about 60 minutes.

EXAMPLE 5 tubes. The following metals are used:

1. Solder (2% Au, 60% Sn, 38% Pb) 2. Babbit Metal (Pb-Bi alloy) 3. Aluminum (Type 3003) 4. Bronze (Tin and copper) (Bunting 13-54) 5. Brass (Zinc and copper) (Anaconda No. 271) 6. Copper 7. Lead The ceramic cores are leached out in heated (9098 C.) percent aqueous acetic acid solutions. In every I case, the cores are completely removed in less than 30 minutes. Tubes are more readily leached than rods. No corrosion or reaction is noticed on any metal and passages of the same diameter as the outer core diameter remain in the metal castings. Using rods or. tubes bent to shapes such as pig tails before firing provides openings in the metal castings of shapes substantially impossible to machine.

EXAMPLE 6 Haynes Stellite Alloy No. 31 (cobalt, chromium and tungsten alloy) is representative of high temperature alloys used, for example, in the production of air foil shapes for turbine engines. The alloy is applied in 10 and 20 mil. (0.25 and 0.50 mm.) coatings as a plasmaspray powder on a zinc oxide tube of Example 3 b pasma-spraymg. Following application of e high EXAMPLE 7 A series of compositions are prepared using 75, 50 and 25 parts of finely divided zircon as filler together with 25, 50 and 75 parts of zinc oxide respectively. The zinc oxide and zircon are wet ball-milled for 2 hours and dried and then each mixture is further mixed with 5.0 parts of wood flour to promote porosity and temporary binders are added in the proportions of Example 1. Rods and tubes are extruded and samples of each are fired at 900, 1000 and 1 100 C. for 6 hours. There are thus obtained samples of rods and tubes representing nine combinations of different firing conditions and different compositions.

All samples fired at 900 or 1000 C. dissolve in hot aqueous 20 percent aqueous acetic acid without difficulty. Of the samples fired at 1 100 C., only the sam ple containing 75 percent zinc oxide dissolves under these conditions. In this composition there is more than 50 percent of zinc oxide present after firing and zircon has reacted completely to zirconia (ZrO and zinc silicate. On the other hand, the composition initially including 25 percent of zinc oxide no longer contains zinc oxide after firing as demonstrated by X-ray analysis.

The composition initially containing 50 percent zinc oxide reacts to an extent such that there is less than 50 percent and even less than 40 percent of zinc oxide in the fired ceramic. This is thus insullficient to permit dissolution of the core.

What is claimed is:

l. A process for providing a cavity of predetermined shape and size in a metallic object in which a core consisting essentially of 40 to about 100 percent uncombined zinc oxide refractory essentially continuous therethrough and replicative of the desired cavity is embedded within the selected metal and thereafter the core is removed by leaching zinc oxide therefrom with aqueous solution ofacid or alkali.

2. 1n the process of claim I the step of leaching zinc j oxide in hot 10 to 20 percent aqueous acid.

3. The process of claim 1 in which leaching is with 33 percent "aqueous potassium hydroxide. 

2. In the process of claim 1 the step of leaching zinc oxide in hot 10 to 20 percent aqueous acid.
 3. The process of claim 1 in which leaching is with 33 percent aqueous potassium hydroxide. 